Field of the Invention
The aspect of the embodiments relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multi-function peripheral having two or more of copy, print, and facsimile functions and a method for controlling the image forming apparatus.
Description of the Related Art
In electrophotographic image forming apparatuses, the surface of a photosensitive member is charged by a charging member and thereafter scanned and exposed by an image exposure member according to image information to form an electrostatic image (electrostatic latent image) on the photosensitive member. Further, a development member supplies toner to the electrostatic image formed on the photosensitive member to form a toner image on the photosensitive member. The toner image formed on the photosensitive member is transferred by a transfer member onto a recording material, such as a sheet, or a transfer-target member, such as an intermediate transfer member.
In recent years, a charging member which is disposed in contact with a photosensitive member and to which a voltage is applied to charge the photosensitive member is widely used because such a charging member is beneficial in reducing ozone and power consumption. Examples of charging methods using such a charging member include a “direct-current (DC) charging method” in which a charging voltage consisting of only a DC voltage (DC component) is applied to the charging member to charge a photosensitive member. The DC charging method is beneficial in that it requires no alternating-current (AC) power source to realize a simpler structure, etc. In the DC charging method, a voltage which is higher than a threshold voltage is applied to the charging member to start charging the surface of the photosensitive member, and the surface potential of the photosensitive member changes substantially linearly to the voltage which is higher than the threshold voltage.
After the photosensitive member passes through a transfer position, the surface potential of the photosensitive member becomes non-uniform due to an effect of a voltage applied to a transfer member. In the DC charging method, the potential leveling effect of AC discharge, which is produced by a method using as a charging voltage a oscillating voltage in which a DC voltage (DC component) and an AC voltage (AC component) are superimposed, cannot be obtained. Thus, in the DC charging method, if the photosensitive member having a non-uniform surface potential after passing through the transfer position is charged and the next image is formed, the uniformity of the surface potential of the photosensitive member after passing through the charging position is often impaired. If the uniformity of the surface potential of the photosensitive member is impaired, the potential difference between the photosensitive member and a developer agent bearing member of a development member varies significantly, and this can cause image defects due to a fog phenomenon, carrier adhesion, etc. The fog phenomenon is a phenomenon in which toner adheres to a non-image portion to which no toner is supposed to adhere, and this phenomenon can occur if the absolute value of the charging potential of the photosensitive member is excessively less than the absolute value of the potential of the developer agent bearing member or if the surface potential of the photosensitive member is reversed with respect to the normal charging polarity. Further, the carrier adhesion is a phenomenon in which carrier of a two-component developer agent which is supposed to be borne on the developer agent bearing member and not supposed to adhere to the photosensitive member adheres to the photosensitive member, and this phenomenon occurs if the absolute value of the charging potential of the photosensitive member is significantly greater than the absolute value of the potential of the developer agent bearing member.
In response to the foregoing situation, Japanese Patent No. 3457083 discusses a pre-exposure member which is provided to irradiate a photosensitive member with light at a position located downstream of a transfer position and upstream of a charging position in the rotation direction of the photosensitive member and eliminate (neutralize) at least some of residual charges of the photosensitive member.
As described above, in order to uniform the surface potential of the photosensitive member having passed through the charging position, the surface of the photosensitive member irradiated with light by the pre-exposure member is charged. When a pre-rotation operation which is a preparatory operation performed before an image is formed is started, if the surface of the photosensitive member between a pre-exposure position and the charging position which is not irradiated with light by the pre-exposure member is charged, the following situation can occur. Specifically, in a case where the surface is a region on which no image forming is performed yet, the back of a recording material can be contaminated due to a fog phenomenon or carrier adhesion caused by an abnormal potential of the photosensitive member. Further, in a case in which the surface is a region on which image forming is to be performed or is performed, the image density can become uneven due to a potential difference (dark decay difference) between a boundary portion irradiated with light by the pre-exposure member and a boundary portion not irradiated with light by the pre-exposure member.
The above-described situation is solved if the application of a voltage to the charging member is started after the surface of the photosensitive member irradiated with light by the pre-exposure member reaches the charging position. In this case, however, the timing to start charging processing is delayed, so the time from the giving of an instruction to start forming an image until the output of the first image, i.e., first copy time (hereinafter, also referred to as “FCOT”), increases.